Before a printer system incorporated in or coupled with a computer or a word processor is initiated into operation, it is necessary to detect the presence or absence of a print sheet storage cassette coupled to each of the sheet supply stages of the sheet supply module and the presence or absence of a print sheet stored in each of the cassettes respectively coupled to the sheet supply stages. Such detection is made immediately after the printer system is initially switched in or each time a new sheet storage cassette is selected an initial sheet storage cassette is selected after the printer system is switched in. For this purpose, a particular order of priority is allocated to the sheet supply stages so that the individual sheet supply stages are successively checked in a sequence complying with such an order of priority.
In a known printer system of the described type, the presence or absence of a print sheet stored in each of the sheet storage cassettes coupled to the sheet supply stages is detected with use of a mechanism by means of which a stock of print sheets stored in the sheet storage cassette of each of the sheet supply stages is to be elevated within the cassette With the stock of print sheets thus elevated in the cassette, a detector lever which forms part of a detector assembly is actuated by the uppermost one of the print sheets within the sheet storage cassette to produce a suitable form of signal to indicate the presence of a print sheet in the cassette If the presence or absence of the print sheets in the sheet storage cassette is detected from the signal produced before the stock of the print sheets is elevated to a predetermined height with respect to the detector lever, it is likely that the sheet storage cassette be determined to have no print sheet stored therein.
Where the printer system is of the type having a plurality of sheet supply stages each including a sheet storage cassette, the detection of the presence or absence of print sheets in each the sheet storage cassettes is performed successively for the individual sheet supply stages in a sequence which complies with an order of priority predetermined for the sheet supply stages as discussed above. If the presence or absence of the print sheets in any of the sheet storage cassettes in such a printer system is detected from a signal produced before the stock of the print sheets is elevated to the predetermined height, it is more likely that the sheet storage cassette be determined to have no print sheet stored therein although there is actually a stock of print sheets stored therein.
The present invention contemplates provision of a sheet supplying apparatus which will eliminate the problem which has thus far been inherent in a printer system of the described type.